1. Field of the Invention
The present invention relates to a semiconductor die package. More particularly, the present invention relates to a folded interposer used to increase the semiconductor die density of a high density semiconductor package.
2. Background of Related Art
As electronic devices, such as cell phones and personal digital assistants (“PDAs”), become smaller, more portable, and more technologically advanced, there is an increasing need for high density semiconductor die packages that can provide the necessary memory for these devices. New, high density semiconductor packages must be easily and cheaply manufactured with existing equipment. In addition, the package must maintain the reliability and quality of the semiconductor die. A semiconductor die package contains many electrical circuit components that must be interconnected to form functional, integrated circuits.
Consumers want their portable devices to perform the same functions as their desktop computers, therefore requiring large amounts of memory in a much smaller electronic device. One way of accomplishing this is to increase the density of a semiconductor die package by using the package's real estate more efficiently. One advantage of high density packaging is that it decreases the length of the connections between the semiconductor die and the package, allowing the semiconductor die to respond faster. Also, reducing the length of the connections reduces the signal propagation time and makes the signal paths less vulnerable to the effects of noise.
Numerous high density semiconductor packages exist in the art. However, these packages are ill-suited for use in small, portable electronic devices because they inefficiently use their real estate, which unnecessarily adds to the overall size of the package. For instance, U.S. Pat. No. 5,128,831 issued to Fox, III et al. teaches a high density package composed of multiple submodules, each of which contains a chip bonded to a substrate. A spacer, which is at least as thick as the chip, is adhesively bonded to the peripheral upper surface of each submodule before the submodules are stacked to form the high density package. The thickness of the spacer causes a gap between each submodule. When multiple submodules are needed, the cumulative effect of these gaps makes the package significantly larger than the size of the components used in the package.
A multichip module comprised of stacked semiconductor dice is disclosed in U.S. Pat. No. 5,323,060, issued to Fogal et al. The semiconductor dice are electrically connected to a substrate by extending long bond wires from bond pads on each semiconductor die to the substrate. In order to accommodate the loop height of the bond wires, a thick adhesive layer is applied between the semiconductor dice. The adhesive layer must be thick enough that the bond wires of the lower semiconductor die do not contact the upper semiconductor die. This multichip module is not suited for small electronic devices because the adhesive layer between the dice increases the overall thickness of the semiconductor package.
U.S. Pat. No. 5,604,377 issued to Palagonia teaches a stack of semiconductor chips designed to be lightweight and to provide better cooling, mechanical shock, and vibration protection. The chips are separated by rigid, insulating interposers formed from a rack structure that contains shelves. The shelves provide electrical insulation and mechanical protection to the chips. The rigid shelves also prevent undue movement of the chips, while the spacing between shelves allows for adequate heat dissipation. Since the shelves are rigid and provide space between the chips, the packaging scheme is not suited for use in small electronic devices.
U.S. Pat. No. 5,818,107 issued to Pierson et al. teaches an integrated circuit package that utilizes metallization features, located at opposite edges of each chip, to attach a stack of chips to a substrate. The chips are bonded together through their metallization features to form a chip stack, which is then bonded to the substrate. The thickness of the metallization features, in addition to the bonding material used, provides a “stand off” or separation between chips. This separation adds to the overall thickness of the integrated circuit package, making it incompatible for use in electronic devices that require small semiconductor packages.
In U.S. Pat. No. 5,994,166 issued to Akram et al., a dense semiconductor package comprising multiple substrates with attached flip-chips is disclosed. The substrates are stacked on top of one another. Column-like connections positioned between the stacked substrates provide electrical communication. The electrical connections must be of sufficient height to provide enough clearance between substrates to mount components and also must be of sufficient strength to provide support between the substrates. Since the column-like connections cause unused space between the substrates, this semiconductor package is incompatible with electronic devices that require small semiconductor packages.
While numerous high density semiconductor packages exist, they share a common disadvantage in that they inefficiently use the space of the semiconductor package. The unused or wasted space may be the result of thick adhesive layers between semiconductor dice or may be caused by rigid interposers or other spacers. Small electronic devices, such as cell phones and PDAs, have very limited space and cannot afford to waste any of this space. Reducing the wasted or unused space in a semiconductor die package is essential because large packages occupy too much of this limited space. It would be preferable to reduce the unused or wasted space in a stack of semiconductor dice by more closely spacing the semiconductor dice. It would be more preferable for the semiconductor dice to be spaced substantially one on top of another. It would be most preferable for the overall size of a high density semiconductor package to be caused only by the thickness of the semiconductor die and a substrate, without substantial thickness coming from additional packaging or unused space.
Methods for connecting dice to a substrate are well known in the art. For example, wire bonding, tape automated bonding (“TAB”), and controlled collapse chip connection (“C4”) are commonly used to physically and electrically connect semiconductor dice to a substrate. Wire bonding utilizes fine wire conductors bonded on one end to the substrate and on the other end to electrical contacts on the semiconductor die. Because wire bonding requires wires to be welded to the die, there must be adequate space to accommodate the wires. TAB utilizes patterned metal on a polymeric tape to join dice together. The joined semiconductor dice are attached to a substrate by outer lead bonding. C4, or flip-chip, bonding uses solder balls on the surface of a semiconductor die to bond the semiconductor die to a substrate.
In addition to the above-mentioned methods, the prior art also discloses using vias to attach a semiconductor die to a substrate and to provide electrical communication between the semiconductor die and substrate. The vias may be filled with conductive metal or flexible leads may be run through the vias to provide electrical communication. As mentioned above, U.S. Pat. No. 5,128,831 issued to Fox, III et al. teaches a high density package composed of multiple submodules, each of which contains a chip bonded to a substrate. Each substrate has a metallization pattern, which comprises multiple conductive traces. A spacer is adhesively bonded to the peripheral upper surface of each submodule before the submodules are stacked. Both the substrate and spacer contain vias that are coincident and substantially coaxial with each other when the package is assembled. The vias are filled with solder to electrically connect the traces of all the submodules. Similarly, U.S. Pat. No. 5,148,266 issued to Kane et al., mentioned in more detail below, uses solid vias to electrically interconnect two chips on opposite sides of a flexible carrier.
U.S. Pat. Nos. 5,252,857 and 5,682,061 issued to Khandros et al. disclose a semiconductor chip assembly containing a semiconductor chip and a substrate that are separated by an interposer. The interposer contains multiple apertures that extend from the first surface to the second surface of the interposer. Flexible leads extending through the apertures are used to connect the chip to terminals on the interposer. The interposer terminals are then connected to contact pads on the substrate. The flexible leads allow for movement of the contacts on the chip relative to the contacts on the substrate, thereby reducing the stresses caused by thermal cycling.
The semiconductor die industry has commonly used flexible components to ameliorate the problems associated with differential thermal expansion of a semiconductor die and substrate. If a die and substrate have different coefficients of thermal expansion, the heat generated by operating an electronic device causes the die and substrate to expand at different rates. When the electronic device is turned off, the semiconductor die and substrate contract at different rates. Over time, these heat cycles place a large amount of mechanical stress on the electrical contacts and solder connections between the semiconductor die and substrate. After repeated cycles, the contacts and connections may fail. The semiconductor die industry has recognized two ways around this problem. First, the mechanical stress on the electrical contacts and solder connections can be minimized by using components that have similar coefficients of thermal expansion. However, this severely limits the types of components that can be used together. A second way around this problem is to incorporate flexible components into the die package. Flexible components known in the art include interposers, circuits, circuit boards, and leads. For example, U.S. Pat. No. 4,851,613 issued to Jacques teaches a flexible circuit board that can be bent, rolled, or folded into a desired shape. The circuit board comprises a substrate, a layer of conductive material in which a circuit is formed, and an insulating layer. Surface mount devices, such as resistors, capacitors, and integrated circuits, can be mounted to the flexible circuit board. Use of the flexible circuit board allows for thermal expansion between the surface mount devices and the circuit board without cracking solder joints or breaking electrical and physical connections.
In U.S. Pat. Nos. 5,148,266 and 5,682,061 issued to Khandros et al., a semiconductor chip assembly containing an interposer and flexible leads is disclosed. The interposer separates a semiconductor chip and a substrate. The chip and substrate electrically communicate through flexible leads that run through apertures in the interposer. The leads connect the chip to terminals on the interposer, which are then connected to contact pads on the substrate. The flexible leads allow for movement of the contacts on the chip and, therefore, reduce the stresses caused by thermal cycling.
U.S. Pat. No. 5,889,652 issued to Turturro teaches an integrated circuit package comprising an integrated circuit attached to a substrate. The substrate includes two portions, a bond portion and a contact portion, separated by a flexible portion. The integrated circuit is attached to the bond portion of the substrate, while the contact portion is attached to a printed circuit board. The flexible portion of the substrate allows for relative movement between the package and the circuit board, minimizing thermal expansion stress on the solder joints.
U.S. Pat. No. 6,002,590 issued to Farnworth et al. teaches a printed circuit board that contains traces attached to a flexible trace surface. Components, such as ball grid array (“BGA”) components, are attached to the traces. The flexible trace surface may be created by the top surfaces of flexible protuberances, which are formed by etching away the substrate not covered by the traces. Alternatively, the flexible trace surface may be formed by depositing a flexible layer onto the printed circuit board. The flexible trace surface allows the traces to he displaced in a direction of thermal expansion of the attached components, thus preventing cracking of solder joints between the trace and component.
U.S. Pat. No. 6,014,320 issued to Mahon et al. teaches a high density circuit module that is comprised of a flex circuit attached to a substrate. The flex circuit is attached to one side of the substrate and folded over to the other side of the substrate. The resulting module includes integrated circuits on one side of the substrate and input/output pads on the opposite side.
While the above-mentioned inventions disclose flexible components in semiconductor die packages, they only disclose attaching one semiconductor die to a substrate. Since high density semiconductor packages are necessary for new generations of electronic devices, it would be preferable to combine flexible components with semiconductor die packages that can accommodate multiple semiconductor dice.
U.S. Pat. No. 5,252,857 issued to Kane et al. discloses both of these features. A dense memory package is disclosed where two memory chips are mounted face-to-face on opposite sides of a flexible carrier or interposer. The two chips contain solder bumps that align when the chips are placed face to face. In addition, the interposer contains pads that are coated with low melting point solder. The bumps on the chips contact the pads on the interposer and are soldered together. Kane also discloses a plurality of pairs of chips mounted on opposite sides of a flexible carrier. The flexible carrier with the attached chips can be folded to connect with substrates, such as printed circuit boards. While Kane discloses a flexible carrier that can be used to connect multiple dice to a printed circuit board or backplane, the pairs of dice are mounted face-to-face on opposite sides of the flexible carrier.
The present invention solves the above-mentioned problems. The present invention discloses a high density semiconductor package that has reduced or eliminated the unused space between stacked semiconductor dice. The resulting high density semiconductor package of the present invention is small and is, therefore, useful in portable electronic devices such as cell phones and PDAs.